Inactivation of Vanadium Bromoperoxidase:  Formation of 2-Oxohistidine^†

The basis of the irreversible inactivation of the vanadium bromoperoxidase (V-BrPO) isolated from the marine alga Ascophyllum nodosum under turnover conditions at low pH (i.e., 15 to 100 mM H₂O₂, 0.1 KBr, ca. 15 nM V-BrPO in 0.1 M citrate, pH 4) has been investigated. Inactivation under these conditions was found to produce 2-oxohistidine as identified by HPLC using electrochemical detection. Formation of 2-oxohistidine requires all the components of turnover (i.e., bromide, hydrogen peroxide, and V-BrPO) as well as low pH; inactivation does not occur nor is significant 2-oxohistidine formed in the presence of hydrogen peroxide alone. The oxidation of histidine did not occur by singlet oxygen generated by V-BrPO, because neither 2-oxohistidine nor inactivation occur under the conditions in which singlet oxygen is produced quantitatively by V-BrPO. The addition of aqueous bromine to N^α-benzoylhistidine at low pH formed N^α-benzoyl-2-oxohistidine. cis-Dioxovanadium(V) (VO₂⁺) in strong acid and MoO(O₂)₂(ox)^2- (ox^2- is oxalate) at pH 5, both of which are functional mimics of V-BrPO by oxidizing bromide by hydrogen peroxide, catalyzed the oxidation of N^α-benzoylhistidine to N^α-benzoyl-2-oxohistidine. Furthermore, when hypobromite was added to N^α-benzoylhistidine in the presence of hydrogen peroxide at neutral pH, conditions under which HOBr would react first with H₂O₂ to produce singlet oxygen, no N^α-benzoyl-2-oxohistidine was formed. Thus the oxidation of histidine in V-BrPO is proposed to occur via oxidized bromine species. Irreversible inactivation V-BrPO was also found to be accompanied by release of vanadium.